While the roles of cysteine as an antioxidant and in cell signaling are widely appreciated, only recently has it been recognized that methionine, like cysteine, functions as an antioxidant and as a key component of a system for regulation of cellular metabolism. The efficiency of methionine as an antioxidant or as a component of signaling systems depends on its ready interconversion between the reduced form (methionine) and the oxidized form (methionine sulfoxide). Methionine sulfoxide reductase catalyzes the reduction of methionine sulfoxide back to methionine. Our studies of methionine sulfoxide reductase A have established: One gene encodes the protein, targeting it to both the cytosol and the mitochondria through two protein initiation sites. The cytosolic form of the reductase is myristoylated, an unexpected covalent modification. The myristoylated protein does not translocate to the membrane, suggesting a new function for myristoylation. Solving the solution structure of the myristoylated enzyme revealed a novel binding pocket for the myristoyl group, the myristoyl nest. We showed that it serves to promote protein-protein interaction. The enzyme was known to be a stereospecific methionine sulfoxide reductase, but a partner oxidase which could form a signaling system had been elusive. We showed that the reductase is bifunctional; it is also a stereospecific oxidase. Overexpression of cytosolic, myristoylated reductase protects the heart from ischemia-perfusion mediated damage. The mechanism of protection has not yet been elucidated.